1. Field of the Invention
This invention relates to a sialon sintered body useful as a material for high temperature structure and a process for producing the same.
2. Description of the Prior Art
Recently, heat-resistant ceramics have widely been developed, and sintered bodies of silicon nitride (Si.sub.3 N.sub.4) are known as the materials excellent in high temperature properties. Since the silicon nitride has a high covalent bond property, it is not easy to be sintered. Therefore, it is difficult to obtain a dense Si.sub.3 N.sub.4 body without sintering additions. Thereupon, a rare earth element oxide such as Y.sub.2 O.sub.3 are added to Si.sub.3 N.sub.4 powder (Japanese Patent KOKOKU No. 56-28865). As the compositions capable of exhibiting strength at a relatively high temperature, there are simultaneous blending of Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 with Si.sub.3 N.sub.4 (Japanese Patent KOKOKU No. 49-21091) and simultaneous blending of Y.sub.2 O.sub.3, Al.sub.2 O.sub.3 and AlN with Si.sub.3 N.sub.4 (Japanese Patent KOKAI No. 59-182276).
On the other hand, sialon has also widely been studied on the premise of blending Y.sub.2 O.sub.3, and it is known that .beta.'-phase (Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z) and O'-phase (Si.sub.2-x Al.sub.x O.sub.1+x N.sub.2-x) have a high oxidation resistance (Non Oxide Tech. Eng. Cerum., pp. 1-30, pp. 105-117, 1986). .alpha.-sialon (M.sub.x (Si,Al).sub.12 (O,N).sub.16) produced from Si.sub.3 N.sub.4, Y.sub.2 O.sub.3 and AlN is also known to be stable in a high temperature region, and its sintering method was reported in detail (Japanese Patent KOKOKU No. 61- 309).
In the above studies, since a rare earth element oxide such as Y.sub.2 O.sub.3 is added, a glass phase of Y.sub.2 O.sub.3 --SiO.sub.2 --Si.sub.3 N.sub.4 --Al.sub.2 O.sub.3, a merrilite phase of Y.sub.2 O.sub.3 --Si.sub.3 N.sub.4 or YAG phase is deposited at grain boundary. When the temperature of these grain boundary phases elevates higher than 1200.degree. C., they soften or creep resulting a sharp decrease in a high temperature strength. Moreover, in the case of blending Y.sub.2 O.sub.3, there is a significant problem in addition to the above decrease in high temperature strength. It is a remarkable inferiority in the oxidation properties of the sintered body. When a sintered body containing Y.sub.2 O.sub.3 is oxidized, SiO.sub.2 -Y.sub.2 O.sub.3 deposits on the surface. This phase does not form a dense membrane, and oxidation proceeds to the inside. When the temperature is elevated over 1500.degree. C., leaching of Y.sub.2 O.sub.3 existing in grain boundary occurs, and sintered body strength sharply decreases.
On the other hand, in the case of the sintered bodies containing 4 elements of Si,Al,O and fundamental elements (Japanese Patent KOKOKU Nos. 51-15850, 51-37656, 51-37657), the above problems caused by Y.sub.2 O.sub.3 are solved. Referring to oxidation properties, since a dense SiO.sub.2 glass layer is formed as the oxidation resistant layer in the sintered body composed of Si--Al--O--N, they are considerably improved.
However, there is still a problem in a high temperature strength in the system composed of Si--Al--O--N. That is, when Al.sub.2 O.sub.3 powder is blended with Si.sub.3 N.sub.4 powder, X'-sialon called pseudo-mullite phase having a weak high temperature strength forms (Yogyo Kyokai-Shi, 93,10,629-635, 1985). As a method for avoiding the production of X'-phase, the inventors noted the mixture state of Al.sub.2 O.sub.3 with Si.sub.3 N.sub.4, and found that the production of X'-phase can be inhibited by using a metal alkoxide as a starting material (Japanese Patent KOKAI No. 61-186266).